voltage chopper

Abstract

The present invention provides an electronic barrier device which may form part of another device such as an isolation barrier or a zener barrier and which includes a voltage limiter such as at least one zener device for limiting voltage in a circuit during a fault condition. voltage, said barrier means comprising crowbar means arranged to latch on said at least one zener means to reduce power dissipation in said at least one zener means in said circuit fault condition, wherein said crowbar means is arranged to latch in response to a change in current sensed in said barrier means.

Description

technical field [0001] The present invention relates to a voltage shunt device, such as also known as a voltage crowbar, and in particular but not exclusively to a voltage shunt device for reducing power dissipation in a potential barrier device Shunt devices, said barrier devices are used in intrinsically safe systems and are found, for example, in isolation barriers or zener barriers. Background technique [0002] Barrier devices typically include a voltage limiter, such as a zener group that provides voltage limitation, a fuse to limit the current in the Zener, and a zener clamp on the output leg of the device to limit the output current to a defined zener clamping voltage. limiting resistor. Zeners and resistors must be used at no more than 2 / 3 of their maximum rating, and fuses must be deemed to remain unbroken at a continuous current of 1.7 times their rating. [0003] However, such an arrangement may prove problematic because under fault conditions, i.e. when the ze...

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Problems solved by technology

Although available zeners may not be dimensioned for this purpose, shunt elements may be

[0049] Also, if in a zener chain, one zener is shorted or low valued, the remaining zeners may be clamped at a lower voltage than the shunt trigger voltage

[0050] Operation of known crowbar arrangements may therefore prove problematic and limiting, since it is desirable to have as high an output voltage as possible with as low a clamping voltage as possible, requiring a specific choice of zener, which may triple the cost

Also, an accurate and stable reference must be used within the voltage crowbar detection circuit, ideally duplexed or tripled, which has further cost implications

Thus, such known voltage-based crowbars are expensive and can prove to be too sensitive since they are configured just around the tolerance limits

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